Positive Eu and Y anomalies and heavy rare earth element enrichments of all lithofacies, along with their low terrestrial element abundance (excluding the shales), indicate their primary precipitation from a mixed hydrothermal-marine environment. In the case of the carbonate itabirites, fluids were CO2 rich. Dolomite precipitated abiotically due to higher temperatures (¡«100 ¡ãC) and high Mg/Ca ratios. The facies changes from carbonate-rich to quartz-rich are attributed to transgression and regression episodes and/or a change in the upwelling hydrothermal fluid composition.
Post depositional reductive dissolution affected the quartz itabirites facilitating the precipitation of ankerite, sideroplesite and magnetite/sulphides from a reducing diagenetic solution, rich in Mn, Mg and Fe, either as lenses or disseminated, in the quartz itabirites and shales. The sideroplesite-richest samples have the most negative ¦Ä13C values (quartz itabirite: ?9.49¡ë; shale: ?5.76¡ë), indicating that at least part of the C is of organic origin, either through the oxidation of organic matter or introduced via the diagenetic fluids. Specularite is replacive and likely related to a post-depositional hydrothermal event.
This study shows that careful mineralogical investigations combined with REE + Y and trace element geochemistry is necessary in order to decipher the depositional environments, which are oxidising, mixed marine-hydrothermal. Post-depositional reductive diagenesis masks in part these primary conditions.